This invention relates to multifocal optical devices which have their multifocal property distributed approximately throughout said devices, and more particularly to multifocal zone plates as described in my patent application Ser. No. 970,571.
A multifocal Fresnel lens has been previously described as a lens consisting of a plurality of annular rings, the surfaces of which are made part of a carrier lens, and have the same inclinations to the optical axis, in a repetitive interleaving pattern, as the individual surfaces of the separate single focal power lenses that they wish to combine.
The concept of a multifocal zone plate was developed as an improvement on the multifocal Fresnel lens, wherein the geometrical spacings of the annular rings of said multifocal Fresnel lens, were set to obey the usual spacing of a zone plate diffraction grating.
Multifocal zone plate mirrors as well as lenses have been described.
However, one can make an improvement in these designs as relates to the level of light intensity available at each of the focal points. Multifocal Fresnel lenses and multifocal zone plates can be designed in such a manner as to cause the intensity of light coming to focus at each focal point, to approach the intensity of light available at the focus of an ordinary monofocal lens.
In order to understand the nature of the problem, we should make note first, that in an ordinary monofocal lens all of the incident light comes to focus in phase at the single focal point. However, in an annular zone multifocal Fresnel lens with two focal points, the incident light is shared between the two focal points. For example, we may have half of the incident light brought to focus in phase at one focal point, and half the incident light brought to focus in phase at the other focal point. However, since brightness is proportional to the square of the convergent in phase light, the intensity of the focussed light at each of the two focii, is only 25% of the usual intensity of light focussed by an ordinary monofocal lens.
The situation is even more dramatic in the case of a zone plate. In this case, the incident light is again reduced at each of the two primary focii. However, the light arriving at each focal point is not in perfect phase and achieves an amplitude of only 1/.pi.th of that at the focal point of an ordinary monofocal lens. Again, since brightness is proportional to the square of the amplitude, the intensity of focussed light at each of the two primary focii of a zone plate is approximately 10% of the usual intensity of focussed light in a monofocal lens.
Finally in the case of a multifocal zone plate, we have a number of different focii, some of them with a 25% image brightness and some of them with a 10% image brightness. However, by judiciously arranging the design of a multifocal zone plate, we can cause some of the zone plate focii to coincide with some of the multifocal Fresnel lens focii. At these double focii we will have an amplitude of light equal to 0.818 (i.e. 1/2+1/.pi.) of that at the focal point of an ordinary monofocal lens. And since brightness is proportional to the square of amplitude, the intensity of focussed light at each of the double focii is approximately 67% of the usual intensity of focussed light in a monofocal lens. FIG. 1 shows the annular zone spacing of such a lens.